The present invention relates generally to illumination devices for controlling a color temperature of light irradiated from an associated light source. More particularly, the present invention relates to an illumination device capable of varying a light quantity for each of a plurality of light-emitting elements based on a desired color temperature, and a controller for use in accomplishing the same.
Conventionally, there is known a psychological effect (called a Kruithof effect) as follows. Bright pale light (i.e., light of a high color temperature) irradiated from a fluorescent lamp of day-white color provides a pleasant atmosphere, but alternatively a gloomy and chilly feeling may result if the luminance (also referred to as flux density, lumens per unit area, light quantity per unit area, or light intensity) of the lamp is too low. Red light (i.e., light of low color temperature) emitted from an incandescent lamp produces a mild atmosphere if the luminance remains low but produces an unpleasant sensation if the luminance is kept too high (see, e.g. FIG. 8). Various kinds of color temperature variable illumination devices capable of varying the color (or color temperature) from a light source have been developed using this psychological effect.
A color temperature variable light-emitting diode (LED) illumination device is known that includes red LEDs, green LEDs and blue LEDs, and a control circuit (or a controller) for driving the respective LEDs of the illumination device and controlling the light quantity (i.e., luminance) thereof. The controller includes individual control settings provided in a corresponding relationship with the respective colors. The color (or color temperature) of illuminating light (or mixed-color light) can be varied by adjusting each of the control settings and separately adjusting the light quantity of each color (red, green or blue). With such an arrangement, it is not particularly easy for the user to set a desired light color (or a desired color temperature).
It would be possible to simultaneously adjust the quantity of the light of different colors through the manipulation of a single control setting. However, the amount of change in the actual color temperature of the produced light does not necessarily coincide with the amount of change in the light color perceived by the human eye. More specifically, even if the amount of change (e.g., 100 K) in a relatively low color temperature (e.g., 2800 K) is equal to the amount of change in a relatively high color temperature (e.g., 4500 K), the change in the relatively high color temperature is hard to perceive while the change in the relatively low color temperature is easy to perceive.
For that reason, if the amount of change in the control setting is merely proportional to the amount of change in the color temperature, a discrepancy occurs between the change in the color temperature adjusted and the change in the color temperature actually perceived. This makes it difficult to use the light-emitting device.
Furthermore, when the color temperatures are same, a psychological effect varies depending on the luminance (i.e., a light quantity with respect to the area to be illuminated), as shown in FIG. 8. It is very difficult for a user to properly adjust the color (color temperature) and the light quantity and achieve a desired psychological effect.
In many aspects it is desirable to use an illumination device which employs an array of light-emitting diodes as a light source instead of the illumination devices (light fixtures) using an incandescent lamp as a light source. However, the incandescent lamp has a feature that, when a luminance ratio is lowered from 100% in a standard lighting context, a light quantity is reduced and a color temperature is also reduced to adjust the chromaticity of illumination depending on a black body locus, as shown in FIG. 9A and the color space chromaticity diagram in 9B.
However, as mentioned above, a user typically sets the color temperature of the mixed-color light by operating each of the three control settings of the controller and separately adjusting the quantity of the red, green and blue light in a conventional illumination device. It is very difficult for the user to adjust the light quantity and the color temperature of illumination to present a chromaticity adjustment feature similar to that of the incandescent lamp.